Many devices have been developed to allow a user to view an image that is to the user's rear. Common examples of such devises are rear-view mirrors in cars and mirror attachments to bicycle helmets. Another similar but somewhat more demanding use involves viewing a rearward image while, for example, rowing a boat. In this situation the traveling direction to the rear of the observer as compared to the front presents further complications associated with the difficulty in making sudden changes of direction. This difficulty makes essentially continuous observation of direction crucial. Typical viewing approaches have involved essentially flat mirrors mounted in the vicinity of the observer, e.g. side or rear-view mirrors in a car and rear viewing contrivances for a bicycle or small boat.
For an apparatus having a simple flat mirror, a viewing angle limited to approximately 15 degrees of arc is not uncommon. This constricted viewing angle presents many inconveniences and even some safety issues. For example, mounting a small mirror on a bicycle safety helmet to the left of the user's left eye (in the case of riding on the right side of the road), blocks as much of the forward view as it provides a rearward image. Additionally, in the unfortunate event of an accident the mirror and supporting structure have the potential for being a dangerous projectile. These difficulties associated with typical bicycle mirrors are certainly not offset by their extremely limited rear viewing angle. Similar considerations are involved with rear-viewing equipment used in small boat applications.
Although the safety concerns previously discussed are not present for rearview equipment in cars, there is a similar limitation on viewing angle. Additionally, for many vehicles such as trucks the rear viewing structure is typically mounted so that it significantly projects from the side of the vehicle. This projection is necessary so that the vehicle itself does not block the line of sight to rearward images. The presence of such structures well beyond the limits of the vehicle's perimeter at times presents a safety hazard to, for example, pedestrians and at the least, often makes negotiating narrow byways very difficult.
Attempts have been made to increase the rearward viewing angle, for example, to between 20 and 45 degrees. One such approach involves de-magnifying the rearward view over the entire range of the image. Although this demagnification provides a larger viewing field, it also leads to a misperception of the distance between the viewer and the object being observed. The resulting difficulties in distance approximation certainly make the act of driving more challenging.
In an attempt to address difficulties with single-mirror approaches, multiple mirror configurations have been employed. Nevertheless, equipment with multiple flat mirrors still have significant problems such as inversion or rotation of the image, reduced image brightness, added complications, and increased physical size. Thus, configurations employing multiple, typically two, cylindrical mirrors have been proposed. For example, one such approach is disclosed in U.S. Pat. No. 4,033,678 dated Jul. 5, 1977. In this patent a rearward viewing device having two cylindrical mirrors is described. The cross-sectional shape of each mirror is specifically adjusted to maintain magnification at essentially unity horizontally across the apparatus while essentially eliminating astigmatism. For example, the use of an elliptical cross section having a focal ratio of two is explicitly exemplified. Despite an asserted improvement in viewing characteristics, the design procedure for accomplishing the specified characteristics involves multiple limitations including a complicated trial and error design process and a limitation of viewing angle.
Therefore, a device that is relatively easy to design as well as fabricate, that allows observation of a relatively wide rearward image, that need not project excessively beyond the user's confines, and that avoids viewing artifacts would be quite desirable.